[Bug c/103881] New: Wconversion false positive when using |= and &= with two rvalues in binary op

[thomas at habets dot se]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103881

            Bug ID: 103881
           Summary: Wconversion false positive when using |= and &= with
                    two rvalues in binary op
           Product: gcc
           Version: 11.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: thomas at habets dot se
  Target Milestone: ---

Got:

Warnings of conversion from int when doing "x |= f() | f()", with all types set
to uint8_t.

No warning when using "=" instead of "|=".

Per https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752, this makes me wonder if
"|=" creates as implicit cast to int (but why?).
But if that's the case then why does "x |= f()" and "x |= a & a" not warn?
And if "|=" implicitly casts to int, that should mean there's no way at all to
not trigger this warning for |= into an uint8_t?

Or does binary "&" implicitly cast to int (again, why?), then why does "x |= a
& a" and "x = f() & f()" not warn?

Reproduce program:
#include<inttypes.h>
uint8_t f();
void foo()
{
  uint8_t t = 0;
  t |= f();       // one rvalue: no warning
  t |= f() & f(); // two rvalues: warn
  t &= f() & f(); // same with &=
  t = f() & f();  // no warning with assign

  uint8_t a = f(); // no warning
  t |= a & a;      // no warning with variables.
  t |= f() & a;    // warning with one rvalue
  t |= a & f();    // no matter position.
}

Output
$ gcc -v -save-temps -Wconversion -c bug2.c
Using built-in specs.
COLLECT_GCC=gcc
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 11.2.0-10'
--with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-11
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-11-R0liEm/gcc-11-11.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-R0liEm/gcc-11-11.2.0/debian/tmp-gcn/usr
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-build-config=bootstrap-lto-lean --enable-link-serialization=8
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.0 (Debian 11.2.0-10) 
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wconversion' '-c' '-mtune=generic'
'-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/11/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu
bug2.c -mtune=generic -march=x86-64 -Wconversion -fpch-preprocess
-fasynchronous-unwind-tables -o bug2.i
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/11/include-fixed"
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-linux-gnu/11/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-linux-gnu/11/include
 /usr/local/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wconversion' '-c' '-mtune=generic'
'-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/11/cc1 -fpreprocessed bug2.i -quiet -dumpbase
bug2.c -dumpbase-ext .c -mtune=generic -march=x86-64 -Wconversion -version
-fasynchronous-unwind-tables -o bug2.s
GNU C17 (Debian 11.2.0-10) version 11.2.0 (x86_64-linux-gnu)
        compiled by GNU C version 11.2.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
GNU C17 (Debian 11.2.0-10) version 11.2.0 (x86_64-linux-gnu)
        compiled by GNU C version 11.2.0, GMP version 6.2.1, MPFR version
4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP

GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 010de8893e7b0de1cd6903c6a1890e7a
bug2.c: In function ‘foo’:
bug2.c:7:8: warning: conversion from ‘int’ to ‘uint8_t’ {aka ‘unsigned char’}
may change value [-Wconversion]
    7 |   t |= f() & f(); // two rvalues: warn
      |        ^
bug2.c:8:8: warning: conversion from ‘int’ to ‘uint8_t’ {aka ‘unsigned char’}
may change value [-Wconversion]
    8 |   t &= f() & f(); // same with &=
      |        ^
bug2.c:13:8: warning: conversion from ‘int’ to ‘uint8_t’ {aka ‘unsigned char’}
may change value [-Wconversion]
   13 |   t |= f() & a;    // warning with one rvalue
      |        ^
bug2.c:14:8: warning: conversion from ‘int’ to ‘uint8_t’ {aka ‘unsigned char’}
may change value [-Wconversion]
   14 |   t |= a & f();    // no matter position.
      |        ^
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wconversion' '-c' '-mtune=generic'
'-march=x86-64'
 as -v --64 -o bug2.o bug2.s
GNU assembler version 2.37 (x86_64-linux-gnu) using BFD version (GNU Binutils
for Debian) 2.37
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/11/:/usr/lib/gcc/x86_64-linux-gnu/11/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/11/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/11/:/usr/lib/gcc/x86_64-linux-gnu/11/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/11/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/11/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wconversion' '-c' '-mtune=generic'
'-march=x86-64'